Polyurethane and polyurethane-urea polymers are belong to the family of thermoplastic elastomers (TPE), and are typical block copolymers comprised of blocks of soft and hard segments. Soft segments form primarily from the polytrimethylene ether glycol and hard segments form primarily from the diisocyanate and chain extenders (the hydroxyl at the ends of the polyether glycols are considered to form part of the hard segment). Polyurethane and polyurethane-urea elastomers are widely used to make fibers, films, foams, resins, adhesives and coatings for various end uses, including automotive bumper covers, solid tires, industrial rollers, shoe soles and sport boots, as well as for biomedical and other applications.
Polyurethanes and polyurethane-ureas containing soft segments from polyalkylene ether glycols generally possess excellent hydrolytic stability, low-temperature flexibility, microbe resistance and rebound properties in addition to good mechanical properties. The polyalkylene ether glycols commonly used are poly(1,2-propylene ether)glycol (“PPG” or “polypropylene ether glycol”) and polytetramethylene ether glycol (“PTMEG” or “PO4G”). Polyethylene glycols (“PEG”), due to their high degree of hydrophilicity, are not widely used in polyurethane industry. The choice of polyalkylene ether glycol to prepare urethane elastomers depends on cost, properties and performance.
The poly(1,2-propylene ether)glycol and polytetramethylene ether glycol soft segments differ in many aspects, for example both of these materials have different chemical structure, reactivity, dihydroxy functionality, molecular weight distribution and crystallinity, and hence, polyurethane elastomers prepared from these soft segments have different property attributes. Poly(1,2-propylene ether)glycol is a low cost polyether glycol which does not crystallize due to the presence of methyl pendant groups in the repeat unit, and has low viscosity and remains in the liquid state at room temperature. It can be handled with ease due to its low viscosity characteristics. However, it has many undesirable attributes including a less reactive secondary hydroxyl at one end, and a significant amount of unsaturation end groups that limits the molecular weight of the elastomers and, therefore, affects the properties. Also, poly(1,2-propylene ether)glycol has limited use in one-shot polyurethane synthesis, and therefore prepolymers are prepared first and then polymerized with a chain extender in the second step.
The molecular weight distribution is much narrower in poly(1,2-propylene ether)glycol, which sometimes can be a disadvantage. For example, S. D. Seneker et al, “New Ultra-Low Monol Polyols with Unique High-Performance Characteristics,” Polyurethane Expo '96, pages 305-313 (1996) (“Seneker”), which is incorporated herein by reference, describes the effect of polyol molecular weight distribution on the properties of polyurethane-urea elastomers where narrow distribution results in elastomers with poor tensile strength properties. It has been demonstrated to have a polyol with broad MWD in order to get good mechanical properties.
Poly(1,2-propylene ether)glycol polymer is also very susceptible to oxidation due to the presence of a tertiary carbon atom in the backbone. As a result, the poly(1,2-propylene ether)glycol-derived polyurethanes and polyurethane-ureas have low oxidative stability.
Recently a new class of poly(1,2-propylene ether)glycol polyether glycol with ultra-low unsaturation ends for improved elastomeric properties have been reported. See, e.g., Seneker. However, these polyether glycols still have the other deficiencies mentioned above.
Polytetramethylene ether glycol is a semi-crystalline, linear polymer with reactive hydroxyl groups at both ends and has functionality of 2. Polyurethane and polyurethane-urea elastomers derived from polytetramethylene ether glycol have superior performance. Unfortunately, polytetramethylene ether glycol is not an ideal soft segment for polyurethane and polyurethane-urea elastomers, and has many undesired attributes. Polytetramethylene ether glycol is a relatively more expensive polymer than poly(1,2-propylene ether)glycol. Also, it is solid at room temperature, has high degree of crystallinity, and has high melt viscosity. Because of high melting temperature and viscosity, this polymer should be stored and transported at above room temperature which is uneconomical and less efficient. In addition, polytetramethylene ether glycol-derived prepolymers and polymers are highly viscous and, as a result, the polyurethane and polyurethane-urea polymers are not easy to process and handle compared to the poly(1,2-propylene ether)glycol-derived elastomers. The polyurethane elastomers derived from polytetramethylene ether glycol show crystallization upon stretching, and polytetramethylene ether glycols exceeding certain molecular weights, usually above 1,800, tend to crystallize in copolymers and thus limits the elasticity of the final polyurethanes and polyurethane-ureas.
In general, the polypropylene ether glycol-derived elastomers are softer and possess high elasticity but poor tensile strength. In contrast, the polytetramethylene ether glycol-derived polyurethanes are harder and have lower elasticity, but higher tensile strength at the same levels of hard segment content. Thus the toughness of the elastomers is not high.
Polyurethane and polyurethane-urea elastomers can also be prepared using polytrimethylene ether glycol (PO3G) to form the soft segment; however, prior to this invention attempts to prepare high performance elastomers have been unsuccessful. For example, the polytrimethylene ether glycols disclosed by Mason in U.S. Pat. No. 3,326,985, which is incorporated herein by reference, were limited to molecular weights of about 1,200 to 1,400 and contained 0.03 to 0.06 milliequivalents/gram of other chain ends including allyl and iodide groups. Attempts to make higher molecular weight required long reaction times and the resultant in polymer had very poor in functionality and was highly discolored.
Polyurethane-urea elastomer compositions derived from polyoxetane polymer are described by S. V. Conjeevaram et al., “Block Copolyurethanes Based on Polyoxytrimethylene Glycols”, Journal of Polymer Science, Polymer Chemistry Edition, Volume 23, pp. 429 to 444 (1985). The polyoxetane polymers obtained from this process is of only academic interest due to the monomer instability, cost and commercial unavailability in large quantities.
It is highly desirable to have elastomers possessing high tensile and high elastic properties as well. It is also highly desirable to have a polyalkylene ether glycol which can overcome most of the above mentioned drawbacks, if not all. For example, a polyalkylene ether glycol ideally should have reactive primary hydroxyls at chain ends, low melting, low viscosity and crystallize at a slower rate for easier handling and processing, and should result in polyurethanes with superior elastic performance while retaining other good mechanical properties.